Lasers, particularly semiconductor lasers with external resonators, are known for use as transmitters for various technical systems, including length and angle measurement systems and interferometers. See, for example, the above noted U.S. Pat. Nos. and corresponding European Patents 90 485, 140 820, 206 661, 206 745, 213 965, and 257 898. In these systems, the function of the external resonator is to control the transmission characteristics of the laser.
It has also known to modulate such laser systems; see for example, the article of E.M. Strzelecki et al. in "Journal of Lightwave Technology", Vol. 6, No. 10, Oct. 1988, pages 1610-1618, where a modulated laser beam for a LIDAR distance measuring apparatus is generated. LIDAR is an acronym for LIght Detection And Ranging, analogous to RADAR, RAdio Detection And Ranging.
However, in all these systems, the laser is the transmitter and the measuring apparatus is a separate unit, so that feedback of the laser beam into the laser is not used for measurement purposes.
R. O. Miles et al. "Journal of Lightwave Technology", Vol. LT-1, No. 1., March 1983, pages 81-9.3, describe a system in which the laser with external resonator is itself used as a sensor and in which one observes the feedback effect of the external resonator on the laser, in dependence upon the parameters to be measured. On page 85 thereof, one sees how, for example, modification of the length of the external resonator leads to periodic fluctuations in the intensity of the light emitted from the laser. However, from observing these fluctuations, one cannot detect the direction of the change in length, so that one does not know whether the external resonator has become longer or shorter or whether, in the worst case, the external reflector is merely vibrating about a rest position and thereby generating intensity fluctuations which are mistaken for a continuous movement. With sensitivities in the nanometer range, this is often the case. Such characteristics severely limit the practical applicability of such sensors. Further, heretofore, only length change and angular tilting has been measured with external resonators.
YAMAMOTO/SHARP U.S. Pat. No. 4,806,778 and corresponding German Patent Disclosure DE-OS 36 06 090 disclose, in FIG. 2, a characteristic direction-dependent signal form of the periodic intensity fluctuations, but this disclosure does not suggest evaluating this signal for direction detection; rather, an amplitude offset is evaluated.